It has been known for some time that a liquid in a container when spun at a constant rotational velocity assumes a parabolic cross-section. The forces on the liquid surface are constant normal to the surface (because of gravity) and vary with the square of the distance radially (centripetal acceleration).
A parabolic surface is ideally suited as an imaging optical device, being perfectly shaped for directing light to a focal point in front of the surface. This principal has been discussed a number of times over the last hundred years or so, and liquid metal techniques are used today by E. F. Borra to make primary mirrors for telescopes using mercury. These mirrors are of high quality, as befits their use in astronomical observations. The unfortunate disadvantages of liquid metal mirrors are obvious: they must be spinning continuously on an axis that is normal to a gravitational field, so they can observe only in one direction; and they cannot be used in space, where gravitational forces are all but eliminated.
The obvious extension of the Borra technique in liquid metal technology is to somehow solidify the liquid while it is spinning so as to preserve the parabolic shape. This process is known as spin casting and is often used to pre-form large, astronomical glass mirrors by slowly cooling a spinning dish of molten glass over a period of months. By spin casting glass mirrors, much of the grinding normally necessary when rough-forming the mirrors is avoided. The spun-cast surface is still imperfect, however, and requires polishing to obtain a dimensionally acceptable surface.
Spin casting has been applied recently to the casting of the world's largest monolithic telescope mirror, one of the 8.4 meter mirrors for the Large Binocular Telescope (LBT). While spin casting of molten glass for telescope-grade reflectors is known, somewhat surprisingly, there are only a few publications describing attempts to spin cast polymeric reflectors. Furthermore, none of these attempts have produced a practical, optically precise plastic reflector.
There are no theoretical reasons why one cannot spin cast very accurate polymeric reflectors using thermosetting plastics. A rotating container of such polymeric material takes on the characteristic parabolic shape, as does any other liquid, and it will cure while spinning. Furthermore, there are a number of advantages to polymeric reflectors. Polymeric reflectors would be considerably lighter in weight than glass mirrors--one-fourth the weight of similarly sized glass mirrors. Plastic composites are also much tougher than glass, being able to withstand much greater physical shocks without cracking or shattering. Both of these properties make plastic mirrors ideally suited for air or space-borne systems where weight is a critical factor and optical components need to withstand sudden accelerations and decelerations. Spin casting techniques also allow for the formation of parabolic optics of extreme curvature and short focal length. Such reflectors, indeed the total optical system, could be housed much more compactly than conventional, longer focal length optical systems, and the payload of the optical system would be reduced as well.
Additionally, very short focal length reflectors have the potential to replace many of the silicon lens systems used on planes and satellites today for infrared imaging. Short focal length, lightweight reflectors are also highly desirable as portable optical devices for use by individuals in the field. Polymeric, parabolic reflectors, mounted in an optical housing with appropriate secondary optics, for example, can be used as terrestrial telescopes or telephoto lenses.
However, in practice, making such mirrors is not a simple matter. Shrinkage and exotherms during polymerization create stresses that deform the surface of the plastic. Curing must be done under very controlled conditions of temperature and atmosphere for best homogeneity. Finally, not all polymers will produce reflectors having a high modulus and strength or a low coefficient of thermal expansion, all of which are necessary in any optical device.
Thus, there remains a need for an effective method for making high quality polymeric reflectors and parabolic reflectors in particular.